The present invention relates generally to a scanner generally attached to a facsimile apparatus and a copier, and more particularly to a sheet scanner which scans an image on a feedable sheet of paper. The sheet scanner differs from a book scanner which scans an image on an unfeedable manuscript such as a book or a thick sheet of paper.
The sheet scanner scans a predetermined image on each paper of a plurality thereof fed one by one. One type of conventional sheet scanner, as shown in Japanese Laid-Open Patent Applications No. 61-27838 and No. 62-264134, uses a feed roller between a separation roller and an eject roller. The separation roller is located at an entrance of a paper supply tray in which a plurality of sheets of paper are to be supplied. The eject roller is located at the entrance of a paper eject tray through which the paper is ejected from the sheet scanner. The paper is fed from the separation roller to the feed roller, and then is fed from the feed roller to the eject roller. These three rollers are located on the feed line through which the paper is fed from the paper supply tray to the paper eject tray. A scanner body which scans a predetermined image on the paper to generate the image data corresponding to the predetermined image is located at a scan line between the feed and eject rollers. The predetermined image comprises a plurality of lines, and the scanner body scans every line. Hereupon, when V.sub.1 is defined as a tangential speed corresponding to the rotation speed of the separation roller, V.sub.2 as that of the feed roller, and V.sub.3 as that of the eject roller, the relationship, V.sub.1 &lt;V.sub.2 &lt;V.sub.3, is established in the above references. Hereupon, the tangential speed corresponds to a speed of feeding the paper. Because of the above relationship, the back end of a first paper does not overlap the front end of a second paper fed subsequently to the first paper s that the papers to be fed are scanned one by one. Incidentally, each roller is always rotated.
However, because of the different tangential speed of each roller, the feeding speed changes at a border between two sections; a first section where the paper is fed by only the feed roller and a second section where the paper is fed by only the eject roller. Hereupon, it is assumed that, when the paper is nipped by the eject roller, the rotation of the feed roller does not affect the rotation of the eject roller so that the paper is fed only by the eject roller in the second section. When the front end of the paper is fed to the scan line, scanning of the predetermined image thereon by the scanner body is started. Each line of the predetermined image on the paper is scanned at a first feeding speed corresponding to the tangential speed V.sub.2 until the front end thereof reaches the entrance of the eject roller. However, when the front end thereof is nipped by the eject roller, then each line of the predetermined image is scanned at a second feeding speed corresponding to the tangential speed V.sub.3. Therefore, the lines in the first section of the predetermined image are scanned at a first feeding speed and those lines in the second section are scanned at a second feeding speed, so that the image to be scanned is blurred at the aforementioned border because a line density included in the image data is not kept constant. The line density is defined as being the scanned lines in a predetermined period divided by the tangential speed at which the paper is fed at that time.
Accordingly, another type of conventional sheet scanner, as shown in Japanese Laid-Open Utility Model Applications No. 63-180646 and No. 64-32839, uses a pulse motor, an oscillation circuit and a motor control circuit. The pulse motor rotates each of the rollers in accordance with the frequency generated by the oscillation circuit. The oscillation circuit generates a plurality of frequencies. In order to prevent the sudden change of the feeding (tangential) speed at the border, the motor control circuit instructs the oscillation circuit to generate a middle frequency between a first frequency corresponding to the first feeding speed and a second frequency corresponding to the second feeding speed.
Still another type of conventional sheet scanner uses an electromagnetic clutch or a solenoid so that each paper is fed at a regular interval and at the same speed from the paper supply tray to the paper eject tray. In this type of sheet scanner, the tangential speed of each roller is preset to the same speed. Incidentally, this sheet scanner further comprises a timing controller which controls the timing of the driving and the halting of the electromagnetic clutch or the solenoid.
However, the aforementioned conventional sheet scanners have the following disadvantages:
1. In the first sheet scanner, because of the three rollers, the construction thereof is complicated and thus this scanner is expensive. In addition, the scanning is blurred because of the sudden change of the feeding speed.
2. On the contrary, the second sheet scanner uses only two rollers so that the construction is simpler than that of the first sheet scanner. However, even in the second sheet scanner, the oscillation circuit and the pulse motor therein, the construction thereof is complicated and thus this scanner is also expensive. In addition, the second sheet scanner cannot completely overcome the blurred scanning because the feeding speed in the vicinity of the border still suddenly changes and the pulse motor cannot compensate for this sudden change.
3. In the third sheet scanner, due to the electromagnetic clutch or the solenoid therein, the construction thereof is complicated and thus this scanner also expensive.